This invention relates to an aqueous ink binder composition, particularly to an aqueous ink binder including (a) 30-60%, by weight based on the dry weight of the binder composition, of a first polymer including, as polymerized units, 15-75%, by weight based on the weight of the first polymer, ethylenically unsaurated carboxylic acid monomer having a pKa from 2.0 to 4.6, the first polymer having a calculated Hansch parameter from 1.3 to 2.1, and (b) 70-40%, by weight based on the dry weight of the binder composition, of a second polymer, the second polymer being an aqueous emulsion polymer. Also the invention relates to an aqueous ink composition including the ink binder and a method for providing an aqueous ink having press resolubility and, as a dry ink film, water resistance.
xe2x80x9cAqueous ink compositionsxe2x80x9d herein include both pigmented inks and unpigmented inks, such as are known in the art as overprint varnishes. Aqueous ink compositions including a soluble resin and an emulsion polymer and the formation of such compositions by blending and by various two-stage polymerization strategies have been disclosed. Such compositions have been posited to offer beneficial ink properties such as rheology. However, the inks formed therefrom have either exhibited unacceptable resolubility on the press or poor water resistance as the dried ink film. When the resolubility of the ink on the press is unacceptable a brief press stoppage permits the ink to dry to an extent that the ink does not redissolve in fresh ink on restarting the press, thereby causing the transfer of dry ink particles to press surfaces and ultimately to the finished printed substrate surface. On the other hand it is desirable that the dried ink film exhibits water resistance so that the finished printed substrate surface is unaffected by contact with water.
U.S. Pat. No. 5,284,894 discloses resin-fortified latex polymers for low-foaming printing inks including a dispersed acrylic polymer, a polyelectrolyte resin, and a nonionic emulsifier.
However, improved aqueous ink binder compositions are still desired in order to provide aqueous inks having both press resolubility and, as the dry ink film, water resistance.
It has now been found that certain ink binder compositions formed from a certain first polymer and an emulsion polymer exhibit the desired performance in that inks formed therefrom exhibit press resolubility and the dry ink films of those inks exhibit water resistance.
In a first aspect of the present invention there is provided an aqueous ink binder comprising (a) 30-60%, by weight based on the dry weight of said binder composition, of a first polymer comprising, as polymerized units, 15-75%, by weight based on the weight of said first polymer, ethylenically unsaturated carboxylic acid monomer having a pKa from 2.0 to 4.6, said first polymer having a calculated Hansch parameter from 1.3 to 2.1, and
(b) 70-40%, by weight based on the dry weight of said binder composition, of a second polymer, said second polymer being an aqueous emulsion polymer.
In a second aspect of the present invention there is provided an aqueous ink comprising an aqueous ink binder and, optionally, a pigment, said ink comprising
(a) a first polymer comprising, as polymerized units, 15-75%, by weight based on the weight of said first polymer, ethylenically unsaturated carboxylic acid monomer having a pKa from 2.0 to 4.6, said first polymer having a calculated Hansch parameter from 1.3 to 2.1, and (b) a second polymer, said second polymer being an aqueous emulsion polymer; wherein the weight ratio of said first plymer to said second polymer on a solids basis is from 0.43:1 to 1.5:1.
In a third aspect of the present invention there is provided method for providing an aqueous ink which has press resolubility and, as a dry film, water resistance comprising
(a) forming an aqueous ink comprising an aqueous ink binder and, optionally, a pigment, said ink comprising (i) a first polymer comprising, as polymerized units, 15-75%, by weight based on the weight of said first polymer, ethylenically unsaturated carboxylic acid monomer having a pKa from 2.0 to 4.6, said first polymer having a calculated Hansch parameter from 1.3 to 2.1, and (ii) a second polymer, said second polymer being an aqueous emulsion polymer;
wherein the weight ratio of said first polymer to said second polymer on a solids basis is from 0.43:1 to 1.5:1; (b) applying said ink to a substrate with a press; and (c) drying, or allowing to dry, said applied ink.
The aqueous ink binder of the present invention includes 30-60%, preferably 35-60%, by weight based on the dry weight of the binder composition, of a first polymer including, as polymerized units, 15-75%, preferably 20-40%, by weight based on the weight of the first polymer, ethylenically unsaturated carboxylic acid monomer having a pKa from 2.0 to 4.6, preferably from 2.5 to 4.5, more preferably from 3.0 to 4.4, the first polymer having a calculated Hansch parameter from 1.3 to 2.1, preferably. By xe2x80x9caqueousxe2x80x9d herein is meant that the medium in which the binder components are dissolved or dispersed is predominantly water. The first polymer includes, as polymerized units, 15-75%, by weight based on the weight of the first polymer, at least one ethylenically unsaurated carboxylic acid monomer having a pKa between 2.0 and 4.6. The pKa of monomers are known in the art and may be found, for example, in Lange""s Handbook (15th Edition). The preferred carboxylic acid monomer is acrylic acid.
The first polymer further includes, as copolymerized unit(s), at least one monoethylenically-unsaturated monomer in addition to the carboxylic acid-functional monomer such as, for example, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, lauryl acrylate, stearyl acrylate, methyl methacrylate, butyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, aminoalkyl (meth)acrylate, N-alkyl aminoalkyl (methacrylate), N,N-dialkyl aminoalkyl (meth)acrylate; urieido (meth)acrylate; (meth)acrylonitrile and (meth)acrylamide; styrene, xcex1-methylstyrene, or other alkyl-substituted styrenes; butadiene; vinyl acetate, vinyl propionate, or other vinyl esters; vinyl monomers such as vinyl chloride, vinylidene chloride, and N-vinyl pyrrolidone. The use of the term xe2x80x9c(meth)xe2x80x9d followed by another term such as acrylate, acrylonitrile, or acrylamide, as used throughout the disclosure, refers to both acrylate and methacrylate, acrylonitrile and methacrylonitrile, or acrylamide and methacrylamide, respectively. Preferred monoethylenically-unsaturated monomers in addition to the carboxylic acid-functional monomer are styrene, xcex1-methylstyrene, and combinations thereof.
The first polymer composition is selected so as to provide a calculated Hansch parameter from 1.3 to 2.1. In the event that more than one first polymer is present in the ink binder composition, a weight-averaged Hansch parameter is used. The Hansch parameter for a polymer is calculated using a group contribution method. The monomer units forming a polymer are assigned a hydrophobicity contribution and the hydrophobicity of the polymer, the Hansch parameter, is calculated based on the weight average of the monomers in the polymer as disclosed in Hansch and Fujita, J. Amer. Chem. Soc., 86, 1616-1626 (1964); H. Kubinyi, Methods and Principles of Medicinal Chemistry, Volume 1, R. Mannhold et al., Eds., VCH, Weinheim (1993); C. Hansch and A. Leo, Substituent Constants for Correlation Analysis in Chemistry and Biology, Wiley, New York (1979); and C. Hansch, P. Maloney, T. Fujita, and R. Muir, Nature, 194. 178-180 (1962). High pH Hansch parameters are used.
Values of the hydrophobicity contributions for several monomers are:
The first polymer is typically substantially soluble in the aqueous ink composition at pH=8; preferably the first polymer is substantially soluble in the aqueous ink composition at pH=7.0. By xe2x80x9csubstantially solublexe2x80x9d herein is meant that 10 parts by weight polymer dissolved in 90 parts of the aqueous medium at 25xc2x0 C. is visually clear.
The first polymer typically has a Mw from 3,000 to 20,000. Mw as reported herein is weight average molecular weight as determined by Gel Permeation chromatography measured vs. polystyrene standards.
The glass transition temperature (xe2x80x9cTgxe2x80x9d) of the first polymer is typically greater than 70xc2x0 C., the monomers and amounts of the monomers selected to achieve the desired polymer Tg range are well known in the art. Tgs used herein are those calculated by using the Fox equation (T. G. Fox, Bull. Am. Physics Soc., Volume 1, Issue No. 3, page 123(1956)). that is, for calculating the Tg of a copolymer of monomers M1 and M2,
1/Tg(calc.)=w(M1)/Tg(M1)+w(M2)/Tg(M2),
wherein
Tg(calc.) is the glass transition temperature calculated for the copolymer
w(M1) is the weight fraction of monomer M1 in the copolymer
w(M2) is the weight fraction of monomer M2 in the copolymer
Tg(M1) is the glass transition temperature of the homopolymer of M1
Tg(M2) is the glass transition temperature of the homopolymer of M2,
all temperatures being in xc2x0K.
The glass transition temperatures of homopolymers may be found, for example, in xe2x80x9cPolymer Handbookxe2x80x9d, edited by J. Brandrup and E. H. Immergut, Interscience Publishers.
The free radical addition polymerization techniques used to prepare the first polymer of the ink binder of this invention are well known in the art. Preferred are polymerization in concentrated solution or in bulk, preferably by a semi-continuous or a continuous process using, for example, a plug flow reactor, a CFSTR, or a hot tube reactor. Free-radical generating initiator compounds such as peroxides and hydroperoxides may be used at a level of 0-5% by weight based on total monomer weight. The monomer mixture may be added uniformly as to rate or composition. The reaction temperature is maintained at a temperature between 100xc2x0 C. and 500xc2x0 C. for a residence time of 1-30 minutes. Preferred is a reaction temperature between 150xc2x0 C. and 260xc2x0 C. for a residence time of 2.5-15 minutes.
The aqueous ink binder of the present invention includes 70-40%, preferably 65-40%, by weight based on the dry weight of said binder composition, of a second polymer which is prepared by aqueous emulsion polymerization. The aqueous emulsion second polymer includes, as (co)polymerized units at least one copolymerized ethylenically unsaturated monomer. Typical ethylenically unsaturated monomers include, for example, (meth)acrylic ester monomers including methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, lauryl acrylate, methyl methacrylate, butyl methacrylate, isodecyl methacrylate, lauryl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate; (meth)acrylonitrile; (meth)acrylamide; amino-functional and ureido-functional monomers; monomers bearing acetoacetate-functional groups; styrene and substituted styrenes; butadiene; ethylene, propylene, xcex1-olefins such as 1-decene; vinyl acetate, vinyl butyrate and other vinyl esters; and vinyl monomers such as vinyl chloride, vinylidene chloride. Preferred are all-acrylic, predominantly acrylic, and styrene/acrylic copolymers.
The second polymer may contain from 0 to 5%, by weight based on the dry weight of the copolymer, copolymerized ethylenically unsaturated acid monomer, based on the weight of the polymer, such as, for example, (meth)acrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, monomethyl itaconate, monomethyl fumarate, monobutyl fumarate, 2-acrylamido-2-methylpropane sulfonic acid, vinyl sulfonic acid, styrene sulfonic acid, 1-allyloxy-2-hydroxypropane sulfonic acid, alkyl allyl sulfosuccinic acid, sulfoethyl (meth)acrylate, phosphoalkyl (meth)acrylates such as phosphoethyl (meth)acrylate, phosphopropyl (meth)acrylate, and phosphobutyl (meth)acrylate, phosphoalkyl crotonates, phosphoalkyl maleates, phosphoalkyl fumarates, phosphodialkyl (meth)acrylates, phosphodialkyl crotonates, and allyl phosphate and maleic anhydride. Preferred is 0-1% copolymerized ethylenically unsaturated carboxylic acid monomer; more preferred is 0-1% copolymerized (meth)acrylic acid.
The second polymer may contain from 0% to 5%, by weight based on the dry weight of the copolymer, copolymerized multi-ethylenically unsaturated monomers such as, for example, allyl methacrylate, butadiene, diallyl phthalate, 1,4-butylene glycol dimethacrylate, 1,2-ethylene glycol dimethacrylate, 1,6-hexanediol diacrylate, and divinyl benzene.
The polymerization techniques used to prepare aqueous emulsion-copolymers are well known in the art. In the emulsion polymerization process conventional surfactants may be used such as, for example, anionic and/or nonionic emulsifiers such as, for example, alkali metal or ammonium salts of alkyl, aryl, or alkylaryl sulfates, sulfonates or phosphates; alkyl sulfonic acids; sulfosuccinate salts; fatty acids; ethylenically unsaturated surfactant monomers; and ethoxylated alcohols or phenols. The amount of such surfactant used is usually 0.1% to 6% by weight, based on the weight of monomer. In a preferred embodiment the emulsion (second) polymer is prepared in the presence of an aqueous solution of the first polymer as the sole surfactant or as a cosurfactant in the polymerization. In a more preferred embodiment the emulsion (second) polymer is prepared in the presence of an aqueous solution of the first polymer at a pH less than 7.5 as the sole surfactant or as a cosurfactant in the polymerization.
Either thermal or redox initiation processes may be used. The reaction temperature is maintained at a temperature lower than 100xc2x0 C. throughout the course of the reaction. Preferred is a reaction temperature between 30xc2x0 C. and 95xc2x0 C., more preferably between 50xc2x0 C. and 90xc2x0 C. The monomer mixture may be added neat or as an emulsion in water. The monomer mixture may be added in one or more additions or continuously, linearly or not, over the reaction period, or combinations thereof.
Conventional free radical initiators may be used such as, for example, hydrogen peroxide, sodium peroxide, potassium peroxide, t-butyl hydroperoxide, cumene hydroperoxide, ammonium and/or alkali metal persulfates, sodium perborate, perphosphoric acid and salts thereof, potassium permanganate, and ammonium or alkali metal salts of peroxydisulfuric acid, typically at a level of 0.01% to 3.0% by weight, based on the weight of total monomer. Redox systems using the same initiators coupled with a suitable reductant such as, for example, sodium sulfoxylate formaldehyde, ascorbic acid, isoascorbic acid, alkali metal and ammonium salts of sulfur-containing acids, such as sodium sulfite, bisulfite, thiosulfate, hydrosulfite, sulfide, hydrosulfide or dithionite, formadinesulfinic acid, hydroxymethanesulfonic acid, acetone bisulfite, amines such as ethanolamine, glycolic acid, glyoxylic acid hydrate, lactic acid, glyceric acid, malic acid, tartaric acid and salts of the preceding acids may be used. Redox reaction catalyzing metal salts of iron, copper, manganese, silver, platinum, vanadium, nickel, chromium, palladium, or cobalt may be used.
Chain transfer agents such as, for example, halogen compounds such as tetrabromomethane; allyl compounds; or mercaptans such as alkyl thioglycolates, alkyl mercaptoalkanoates, and C4-C22 linear or branched alkyl mercaptans may be used to lower the molecular weight of the emulsion polymer and/or to provide a different molecular weight distribution than would otherwise have been obtained with any free-radical-generating initiator(s). Linear or branched C4-C22 alkyl mercaptans such as n-dodecyl mercaptan and t-dodecyl mercaptan are preferred. Chain transfer agent(s) may be added in one or more additions or continuously, linearly or not, over most or all of the entire reaction period or during limited portion(s) of the reaction period such as, for example, in the kettle charge and in the reduction of residual monomer stage.
In another aspect of the present invention the emulsion polymer may be prepared by a multistage emulsion polymerization process, in which at least two stages differing in composition are polymerized in sequential fashion. Such a process usually results in the formation of at least two mutually incompatible polymer compositions, thereby resulting in the formation of at least two phases within the polymer particles. Such particles are composed of two or more phases of various geometries such as, for example, core/shell or core/sheath particles, core/shell particles with shell phases incompletely encapsulating the core, core/shell particles with a multiplicity of cores, and interpenetrating network particles. Each of the stages of the multi-staged emulsion polymer may contain the same monomers, surfactants, chain transfer agents, etc. as disclosed herein-above for the emulsion polymer. In the case of a multi-staged polymer particle the calculated Tg, the amount of optional monomer or the calculated Hansch parameter for the purpose of this invention is to be determined from the overall composition of the emulsion polymer without regard for the number of stages or phases therein. The polymerization techniques used to prepare such multistage emulsion polymers are well known in the art such as, for example, U.S. Pat. Nos. 4,325,856; 4,654,397; and 4,814,373.
The glass transition temperature (xe2x80x9cTgxe2x80x9d) of the emulsion polymer is from xe2x88x9280xc2x0 C. to 120xc2x0 C. Tgs used herein are those calculated by using the Fox equation as described herinabove. The Hansch parameter of the emulsion polymer is preferably greater than 3.0, more preferably greater than 3.5. The average particle diameter of the emulsion copolymer particles is preferred to be from 30 nanometers to 500 nanometers, as measured by a BI-90 Particle Sizer.
In the second aspect of the present invention there is provided a pigmented or unpigmented aqueous ink suitable for, for example, flexographic, gravure, or offset printing processes. The ink includes an aqueous ink binder and, optionally, one or more pigments including (a) a first polymer including, as polymerized units, 15-75%, by weight based on the weight of the first polymer, ethylenically unsaurated carboxylic acid monomer having a pKa from 2.0 to 4.6, the first polymer having a calculated Hansch parameter from 1.3 to 2.1, and (b) a second polymer, the second polymer being an aqueous emulsion polymer; wherein the weight ratio of the first polymer to the second polymer on a solids basis is from 0.43:1 to 1.5:1, preferably from 0.53:1 to 1.5:1.
The optional pigment is typically dispersed in an aqueous medium by admixing an aqueous medium; a pigment; and a pigment dispersant, as known in the art, and subjecting the admixture to shear for a time sufficient to disperse the pigment. The pigment may be at least one organic or inorganic pigment, a pigment composite, or mixtures thereof. The admixture includes 1-50%, preferably 8-25%, by weight, based on dry pigment weight, of a pigment dispersant. In a preferred embodiment the first polymer of this invention is used as a part, preferably a predominant part, more preferably all, of the pigment dispersant. In an ink containing more than one first polymer the preferred Hansch parameter, on a weight fraction-averaged basis, of all of the first polymer(s) in the ink is from 1.3 to 2.1, more preferably from 1.4 to 2.0, most preferably from 1.5 to 1.9.
The admixture is then subjected to shearing in a grinding or milling device as is well known in the pigment dispersion art. Such grinding or milling devices include roller mills, ball mills, bead mills, attrittor mills and include mills in which the admixture is continuously recirculated. The shearing of the admixture is continued for a time sufficient to disperse the pigment by which is meant herein that the objective of reducing the pigment to its primary particle size to maximize stability and color development is typically balanced against the cost in time and energy required to achieve the ultimate extent of dispersion. The time sufficient to disperse the pigment is typically dependent on the nature of the pigment and pigment dispersant and the grinding or milling device which is used and will be determined by the skilled practitioner. The pigment dispersion is typically provided at 30-50% pigment, by weight, based on the weight of the pigment dispersion. The pigment dispersion may also include additional ingredients such as waxes, defoamers, biocides, other binder resins, thickeners, humectants, and the like or they may be added to the ink later. The ink binder and the pigment dispersion are then admixed with low shear mixing.
In the third aspect of the present invention there is provided a method for providing an aqueous ink which has press resolubility and, as a dry film, water resistance including
(a)forming the aqueous ink of the present invention; (b)applying the ink to a substrate with a press such as, for example a flexographic, gravure, or offset printing press; and (c)drying, or allowing to dry, the applied ink.
The following examples are presented to illustrate the invention and the results obtained by the test procedures.
Abreviations used throughout:
STY=styrene
AMS=xcex1-methyl styrene
MMA=methyl methacrylate
BA=butyl acrylate
EHA=2-ethylhexyl acrylate
AA=acrylic acid
DI water=deionized water